Chemical vapor deposition (CVD) and physical vapor deposition (PVD) techniques are today routine technologies for the deposition of thin single and multi-layer films. These thin films are commonly used for optical, electrical, ornamental, wear resistant, and decorative applications. The films allow very small amounts of material to be deposited onto a substrate. These films allow not only a minimum of otherwise expensive materials to be used, but also enable miniaturization, enhancement of optical properties, precise electrical performance, user defined structures/morphology, and a profusion of surface specific compositions and surface treatments. However, when these conventional deposition techniques are used to deposit multiple components into a single film, the resulting film usually exhibits the lesser properties of the two components. For example, multi-layer structures which include one hard layer such as TiN (titanium nitride), and one soft layer such as gold (Au), do not exhibit the strength of TiN. Rather they are limited by the mechanical adhesive strength of the gold layer. At the same time the desired optical or electrical properties of gold are diminished due to the TiN phase. It is primarily to these difficulties in thin film fabrication methods that the present invention is directed. Other novel structures made possible by the method of the invention, including unique single component anisotropic thin films and post method deposited component films, will be further described below.
It is therefore a primary object of the present invention to provide a method for fabricating three dimensional, anisotropic nanostructured thin films.
A further object of the invention is to provide a method for fabricating nanostructured wear resistant thin film coatings with anisotropic properties.
An additional object of the invention is to provide a method for fabricating sculptured nanostructures.
Still another object of the invention is to provide a method for fabricating gold nanostructured thin film coatings with anisotropic electrical conductivity properties.
Yet another object of the invention is to provide a method for fabricating nanostructured composite conductive films.
These and other objects are obtained with the present invention of a method for fabricating three-dimensional anisotropic thin films.
The term xe2x80x9cNanostructurexe2x80x9d as used herein refers to structures with dimensions in a range between 1 to less than 100 nanometers (nm), as presently defined in the industry, and only defining the thickness range of the anisotropic lattice depositions produced by the method of the invention without restricting the ability for additional stratification or film growth.
Thin films of this type are commonly deposited onto a substrate, either in single or multiple layers, by means of chemical vapor deposition and physical vapor deposition techniques. A typical physical vapor deposition involves placing an object to be coated or upon which a film is to be deposited (substrate) within a vacuum chamber which also contains one or more target materials. A vacuum pump then produces a vacuum within the chamber. A small quantity of a gas, as, for example, argon, is admitted to the chamber. A power source such as a DC magnetron, or an RF diode power source, or an RF magnetron is then turned on, which causes the Ar (argon) to ionize and ion bombard the target material, which causes sputtering of the target material with subsequent sputtering deposition of the target material onto the substrate. The described method is also applicable to reactive depositions. These PVD techniques have found wide applications in micro electronics, decorative coatings, machine tool coatings, and other uses far too numerous to enumerate.
Prior utilization of this thin film technology has yielded either multi-layered, non-structured, or quantum dot depositions which will be more fully explained. In each case, these non-structured films at best provide some degradation of desirable original qualities for a two or more component film. Very thin films buckle due to the stress of having lattice structures slightly different in size from those of the materials upon which the films are grown. Just a few percent difference in lattice size creates stresses or pressures in a film that can reach up to 105 torr. These huge pressures, when a new layer is deposited on the top of the first one, force the initially flat film to separate into dots and then top up into the third dimension to relieve stress. It occurred that rather than designing around this problem as was the case in the past, it might be possible to control this phenomenon, and have consecutive depositions yield three dimensional, anisotropic thin films which maintain the original desired properties of each of the deposited materials.
In the present invention the above described deposition techniques are employed to fabricate a three dimensional, anisotropic thin film. What is meant by this is that a substrate, which can be viewed for the sake of simplicity as a planar substrate in a horizontal position, has two or more different materials deposited on its surface. Alternate sequential depositions of the target materials cause each one to be deposited initially as discrete, separate, individual dots. Each additional sequential deposition of the same material then builds up on top of each initial dot of the material, so that the resultant film is composed of two or more multi-columns of each material, the columns being substantially vertical and normal to the planar substrate.
To this end a standard physical vapor deposition apparatus was modified so as to have precise control over the parameters of PVD deposition by providing accurate control of the partial pressure of plasma forming and reactive gases; spectral control of the plasma, in particular gas characteristics and wave length; accurate control of input gas flows, including real time feed-back between the control system and feed gages; real time feed back on deposition parameters; accurate control of gun energy and time; and accurate control of substrate temperature.
A method has been devised making use of the above control parameters that, when combined with the tendency of sputtered material to initially nucleate in small dots forming discontinuous islands with a specific area coverage ratio, which in turn creates a controllable three dimensional structure. At least two targets (e.g. gold and titanium), whose vacuum sputtering forms a characteristic nano-scale dot formation are used to form a discontinuous island film formation. This island film formation represents a certain ratio of area coverage versus open and less-conductive spaces between those islands. It is the control of this normal tendency of island formation (nucleation), based on specific and well known material characteristics, that this process is based. This natural structure characteristic is inherent in all sputtered material, which represents, in general, their lowest energy level and their negative affinity to each other. A plasma forming gas (e.g. argon) is introduced into the vacuum chamber, and a reactive gas (e.g nitrogen) may also be introduced into the vacuum chamber to react with at least one of the target materials. Gas metering is accurately controlled with a gas flow control system (e.g. spectrophotometer, mass flow meter, optical flow control, and other suitable gas flow control systems). In addition, gas flow and gas composition within the chamber is further refined with a real-time feed-back system so as to have accurate control over the partial pressure of the plasma forming and reactive gases. Accurately controlled and timed gun voltage (the power supply to the targets), together with substrate temperature control, with real-time feed-back on deposition parameters, and with the two or more target materials being sputter deposited in consecutive order, permits the formation of a three dimensional anisotropic structure. This unique, new structure preserves the original qualities of each one of the target materials. This structure is in a basic relevant contrast with the classical multi-layered deposition systems that only display the dominant material characteristics.
In general the success (S) of the deposition of the structured film can be expressed by the following empirical equation:
S=F(N, V1dep, V2dep, tdep, A) 
Where N is the concentration of forming nuclei, V1dep is the rate of deposition of the fist material, V2dep is the rate of deposition of the second material, tdep is the time of deposition, and A is the affinity of these materials to each other. Optimal conditions for the deposition of the structured films are in the range:
X1 less than S less than X2 
In the case of X1 we will have a chaotic mixture (or alloy) non-structured film. In the case of X2 we will obtain a multi-layer deposition. With the controlled conditions of deposition described herein, which includes the probability of nucleation, rates of deposition, time of deposition and affinity of the two major components, a three dimensional anisotropic structure is obtained. While examples including gold, and titanium are given, it is to be understood that the method is applicable to any material capable of being sputtered deposited within the parameters of the method of the present invention. Other materials include, but are not limited to Pd, Al, Cu, and C (diamond). The xe2x80x9cmatrixxe2x80x9d of the invention can be created, as one available approach, by reactive deposition of metals, which can be a variety of compounds, such as but not limited to: nitrides (ZrN, CrN, TiAlN);carbo-nitrides (TiCN); carbides (BC, SiC, WC); and oxides (Al2O3, Zr O2) as well as by non-reactive deposition of other hard metals such as NiCr